Largest Humpback Whale Gathering Witnessed; Worrying Signs for Long-Term Food Supply
29.04.2011 - Water & Oceans, Ice & Snow, Flora & Fauna, Antarctic
Findings recently published in the journal PLoS ONE have shed new light on an extreme form of krill behaviour and the response of humpback whales which prey on them. The findings also provide a warning to long-term sustainability of krill populations – a staple of humpback whales’ diet – in light of diminishing sea ice off the coast of the Western Antarctic Peninsula due to rapid warming in the region.
Until recently, scientists believed that humpback whales fed in the Antarctic during summer until krill moved to relocate under the sea ice in the austral autumn, after which the whales would migrate to warmer breeding grounds, where some of the highest humpback densities had been observed. However during visits to Wilhelmina Bay on the Western Antarctic Peninsula in 2009 and 2010 aboard the research vessel Lawrence M. Gould, scientists from the University of Massachusetts Boston and Duke University in Raleigh, North Carolina discovered that their previous suppositions on whale feeding and migration behaviour needed a revision.
Having ventured into Wilhelmina Bay in May 2009 and May 2010 later into the austral autumn than previous expeditions had ever ventured, the team of scientists discovered the highest humpback whale density ever witnessed along with the largest and densest swarm of krill documented in more than 20 years. The team counted 500 humpbacks in the bay at an average density of 5.1 whales per square kilometre.
Wilhelmina Bay has been unusually ice-free over the past few years – a phenomenon scientists attribute to rapid warming the region has been experiencing over the past few decades – which has made it easier for humpbacks to prey on krill, which normally use sea ice as a refuge from predators. The humpbacks are sticking around later than usual into the winter before they migrate to warmer waters in this particular region off the Western Antarctic Peninsula due to easier access to a larger number of krill, and some humpbacks may even stay throughout the winter. The researchers even detected male humpbacks singing, an act normally found in breeding grounds in warmer waters.
A little further away, the researchers found a smaller yet equally dense krill swarm in a nearby bay along with more humpbacks. In yet another bay, a glut of whales prevented them from even lowering their acoustic equipment into the water. On a return trip to Wilhelmina Bay in May 2010, the scientists found krill and whales in amounts comparable to 2009.
While access to larger numbers of krill might be good news for humpbacks on the short term, it could create difficulties for whales on the longer term, as krill also use sea ice as a place to spawn, and diminishing krill populations over the long-term have been attributed to retreating sea ice. Despite the size of current krill swarms, the areas where they are currently found in Wilhelmina Bay and elsewhere are likely to see a decline in krill numbers.